An interferometer used, for example, in a spectroscopic unit of a Fourier-transform infrared spectroscopy (referred to as “FTIR” hereinafter) is intended that, light emitted from an infrared light source is split into two light beams. One of the two light beams is reflected by a fixed mirror, while the other is reflected by a reciprocal moving mirror. Then, these light beams are synthesized again to thereby generate an interference wave by an optical path difference.
In this FTIR, the intensity of the synthesized light by this interferometer draws sine curves every different wavelength, when the moving mirror is continuously moved. The synthesized light in this interferometer is spectrally dispersed through Fast Fourier Transform (FFT) to thereby obtain the light intensity of every wavelength (spectrum). Thus, it is adapted to analyze a composition ratio and concentration of a measuring object.
At this time, it is required to measure the interference waveform with accuracy conformable to the FFT operation in this interferometer. It is indispensably necessary to observe a position of the moving mirror with high accuracy.
Thus, the moving mirror is equipped on a predetermined movement mechanism. The moving mirror is configured to be reciprocated at a constant speed in a predetermined section by controlling the movement mechanism with a movement control part.
The movement mechanism is configured of, for example, a cart on which the moving mirror is mounted, a linear guide for supporting the cart reciprocally, and a linear actuator such as a voice coil motor for reciprocating the cart, and the like.
The movement control part is adapted to feedback-control the position of the cart (and the moving mirror). The movement control part is followed by measuring the position of the cart by a position sensor such as a laser displacement measurement system, and generating control signal to the linear actuator so that the measurement position follow to a target position. Since this target position is reciprocated at a constant speed, it is set to depict a triangular waveform (also, referred to as “target position triangular waveform” hereinafter), when a horizontal axis represents a time as shown in FIG. 1.
In such a system, although a constant speed reciprocal movement is targeted, it is necessary that the cart and the moving mirror should be turned back, while being decelerated and accelerated in the around of an apex of the target position triangular waveform at which a forward movement and a backward movement are switched. Therefore, there occurs a turn-back section in which the constant speed cannot be maintained as shown in FIG. 1.
Meanwhile, in recent years, there arises an increased demand for shortening a measurement time, and it becomes necessary to speed up the reciprocal movement of the mirror accordingly.
However, if the reciprocal movement is speeded up, a thrust force at a timing of turn-back is significantly increased. There is concerned a problem that an excessive load is applied to the movement mechanism of the mirror, which results in short life thereof and increasing frequency of breakdowns. For example, If two or three times faster than conventional one is required, but the maximum instantaneous thrust force required for the turning back of the mirror also becomes two or three accordingly. Therefore, deformation of the voice coil constituting the movement mechanism and wear of a steel ball of the linear guide become remarkable, results in negative influence on the product lifetime are caused and the breakdowns become easily occur.
Further, since the time required for turning back the cart becomes shorter in accordance with the faster speed, the time elapsed from the turned of the cart up to the movement at a constant speed becomes also shorter. It results in that speed stability of the cart is deteriorated, but also the measurement accuracy is deteriorated.